Laser-based underwater frequency transfer with sub-picosecond timing fluctuation using optical phase compensation.

Area of Science:

• Optical Physics
• Metrology
• Underwater Communication

Background:

• Accurate frequency dissemination is crucial for scientific and technological applications.
• Underwater environments pose significant challenges for signal transmission due to signal degradation and instability.
• Existing methods for frequency transfer are limited in underwater applications.

Purpose of the Study:

• To demonstrate a sub-picosecond laser-based underwater frequency transfer system.
• To analyze and measure the timing fluctuation and instability of the transferred radio-frequency signal.
• To evaluate the potential of this technique for underwater atomic clock transfer.

Main Methods:

• Utilized a sub-picosecond laser system for frequency transfer.
• Implemented optical phase compensation to mitigate underwater signal distortion.
• Disseminated a 500 MHz radio-frequency signal over a 5-m underwater link for 5000 seconds.

Main Results:

• Achieved a total root-mean-square (RMS) timing fluctuation of approximately 162 femtoseconds (fs).
• Measured a fractional frequency instability of 2.8 × 10^-13 at 1 second and 2.7 × 10^-16 at 1000 seconds.
• Demonstrated performance superior to current commercial Cesium (Cs) or Hydrogen-master clocks in terms of instability.

Conclusions:

• The developed laser-based underwater frequency transfer technique is highly stable and accurate.
• This method shows significant potential for transferring atomic clock signals in aquatic environments.
• The achieved instability is competitive with, and in some cases surpasses, state-of-the-art atomic clocks.